1. Field of the Invention
This invention relates to a method of and system for erasing a radiation image remaining on a stimulable phosphor sheet after the stimulable phosphor sheet is exposed to stimulating rays in order to read out the radiation image stored thereon.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
Further, there has been proposed various methods of processing the electric image signal, before it is used for reproducing the radiation image of the object, so that the visible image thus produced has an improved image quality, which makes it an effective tool when illnesses must be efficiently and accurately diagnosed. (See Japanese Unexamined Patent Publication No. 56(1981)-11395, and U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, and 4,387,428 and the like.) According to the teachings of those patent publications, it is preferred that a stimulable phosphor which emits light of 300 to 500 nm upon stimulation by stimulating rays of 600 to 700 nm be used in order to separate the wavelength range of the light emitted by the stimulable phosphor sheet from that of the stimulating rays and to detect at high efficiency the light emitted by the stimulable phosphor sheet which is very weak.
In view of economy, it is preferred that the stimulable phosphor sheet be repeatedly reused. Strictly speaking, the stimulable phosphor sheet is used in various forms, (e.g., in the form of a drum or in the form of a panel). However, in this specification, all the recording media having a stimulable phosphor layer will be referred to as "the stimulable phosphor sheet".
Though the radiation energy stored on the stimulable phosphor sheet during exposure to radiation should be completely released if the stimulable phosphor sheet is exposed to stimulating rays of a sufficient intensity during read-out of the radiation image, actually the radiation energy cannot be completely released only by exposure to the stimulating rays. Thus there arises a problem that, when the stimulable phosphor sheet is reused, part of the radiation energy stored on the stimulable phosphor sheet upon a radiation image recording is not released during exposure to the stimulating rays and can result in noise of the radiation image recorded in the following radiation image recording.
Further since a trace amount of radioisotopes such as .sup.226 Ra, .sup.40 K and the like are included in the stimulable phosphor, radiation energy accumulates in the stimulable phosphor sheet due to radiation emitted by such radioisotopes even if the stimulable phosphor sheet is left as it is. The radiation energy which accumulates in the stimulable phosphor sheet while it is left as it is (will be referred to as "fog", hereinbelow) can also result in noise of the radiation image recorded in the following radiation image recording.
In order to prevent generation of noise due to the remaining part of the radiation energy and the fog, this applicant has proposed methods of erasing the stimulable phosphor sheet in which the stimulable phosphor sheet is exposed to erasing light containing light having wavelengths within the stimulating wavelength range so that the radiation image remaining on the stimulable phosphor sheet is sufficiently released prior to the following radiation image recording.
In one of such methods, a light source which emits relatively long wavelength light ranging from visible light to infrared light, e.g., a tungsten lamp, a halogen lamp and an infrared lamp, is used as the source of the erasing light (U.S. Pat. No. 4,400,619). In another method, a light source which emits relatively short wavelength light ranging from 400 to 600 nm, e.g., a fluorescent tube, a laser, a Na-lamp, a Ne-lamp, a metal halide lamp, a Xe-lamp, is used as the source of the erasing light (U.S. Pat. No. 4,496,838). In still another method, after the stimulable phosphor sheet is once exposed to erasing light, the stimulable phosphor sheet is again exposed to erasing light at an intensity of 1/5 to 3/10000 of the intensity of the erasing light in the first erasure immediately before it is reused (U.S. Pat. No. 4,439,682). It is said that most efficient erasure takes place when visible range light is used as the erasing light.
However, when erasure is effected by the use of erasing light containing therein no ultraviolet range light, remaining radiation energy in the form of relatively deep trapped electrons which is difficult to release by visible light cannot be sufficiently released. On the other hand, when erasure is effected by the use of erasing light containing a large quantity of ultraviolet range light, the ultraviolet range light itself produces other trapped electrons and accordingly the radiation energy cannot be sufficiently released though the remaining radiation energy in the form of relatively deep trapped electrons can be released.
Thus, it is very difficult to release both the radiation energy in the form of deep trapped electrons and the radiation energy in the form of normal trapped electrons at one time and effectively release the remaining radiation energy. Accordingly, in the present state, influence of the remaining radiation energy cannot be satisfactorily avoided especially when the high-sensitive recording is effected on a reused stimulable phosphor sheet.